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Papers by Paul Salvador

Journal of the American Ceramic Society, 2019

Combinatorial substrate epitaxy (CSE) was used to study the orientation relationships (ORs) and p... more Combinatorial substrate epitaxy (CSE) was used to study the orientation relationships (ORs) and polytypic stability of AEMnO 3 (AE = Ca, Sr) thin films grown on polycrystalline SrMnO 3 and SrTiO 3 substrates. SrMnO 3 films with the stable four-layered hexagonal (4H) and metastable three-layered cubic (3C) structures were also grown on (111) and (100) SrTiO 3 single crystal substrates, respectively. Electron backscatter diffraction data were used to determine the following ORs, which hold true regardless of the substrate surface orientation: (001)[100] 4H SrMnO 3 ||(001)[100] 4H SrMnO 3 , (111)[110] 3C CaMnO 3 ||(001)[100] 4H SrMnO 3 , and (001)[100] 4H SrMnO 3 ||(111)[110] 3C SrTiO 3. These are all simply the eutactic OR, which aligns the eutactic planes and directions; its ubiquity indicates that the interface energy is generally lower for the eutactic OR than for all other possible ORs. 3C SrMnO 3 was found to grow only on very near (100) 3C SrTiO 3 grains. This narrow range of epitaxial stabilization suggests that the penalties of higher interfacial and/ or strain energies between polytypic perovskites adopting the eutactic OR are not significant enough to overcome the volumetric formation energy of the stable phase in these growth conditions, except for very special orientations. K E Y W O R D S characterization, interfaces, orientation relationships, thin films | 2233 ZHOU et al. substrate driven metastable 4H CaMnO 3 further supports this conclusion. The combined results, however, support the idea that CSE can quickly unravel the nature of epitaxial growth and competitive polymorph nucleation, and should allow for computational predictions of epitaxial growth.

ECS Transactions, 2009

We report on the surface topography, electron tunneling and chemical characteristics of 10nm- and... more We report on the surface topography, electron tunneling and chemical characteristics of 10nm- and 50nm-thick La0.7Sr0.3MnO3 (LSM) thin-films at temperatures up to 580oC in 10-3 mbar oxygen pressure, using in situ scanning tunneling microscopy and spectroscopy (STM/STS). Coarsening of the grains was found on the 10nm-thick LSM film. High resolution mapping of the tunneling spectra onto the surface topography showed a higher electron exchange rate at select grain boundaries compared to grain surfaces. A threshold-like drop in the tunneling current was observed at positive bias in STS, and is suggested as a unique indicator to the activation polarization in cation-oxygen bonding on LSM. Sr-enrichment and Mn-depletion were found on the surface at high temperature using Auger electron spectroscopy, accompanied by a reduction in tunneling conductance in STS. This suggests that the Mn-terminated surfaces are more active for electron exchange in oxygen reduction compared to the (La,Sr)-term...

MRS Proceedings, 2008

Using a model cathode-electrolyte system composed of epitaxial thin-films of La1-xSrxMnO3-δ (LSM)... more Using a model cathode-electrolyte system composed of epitaxial thin-films of La1-xSrxMnO3-δ (LSM) on single crystal yttria-stabilized zirconia (YSZ), we investigated changes in the cation concentration profile in the LSM during heating and under applied potential using grazing incidence x-rays. Pulsed laser deposition (PLD) was used to grow epitaxial LSM(011) on YSZ(111). At room temperature, we find that Sr segregates to form Sr enriched nanoparticles and upon heating the sample to 700°C, Sr is slowly reincorporated into the film. We also find different amounts of Sr segregation as the X-ray beam is moved across the sample. The variation in the amount of Sr segregation is greater on the sample that has been subject to 72 hours of applied potential, suggesting that the electrochemistry plays a role in the Sr segregation.

Microscopy and Microanalysis, 2014

MRS Proceedings, 2000

ABSTRACTThin films of NdNiO3 were grown using pulsed laser deposition on single crystal substrate... more ABSTRACTThin films of NdNiO3 were grown using pulsed laser deposition on single crystal substrates of [100]-oriented LaAlO3 and SrTiO3. X-ray diffraction and reflectivity, scanning electron microscopy, and atomic force microscopy were used to characterize the chemical, morphological and structural traits of the thin films. Single-phase epitaxial films are grown on LaAlO3 and SrTiO3 at 625°C in an oxygen pressure of 200 mTorr. At higher temperatures, the films partially decompose to Nd2NiO4 and NiO. The films are epitaxial with the (101) planes (orthorhombic Pnma notation) parallel to the substrate surface. Four in-plane orientational variants exist that correspond to the four 90° degenerate orientations of the film's [010] with respect to the in-plane substrate directions. Films are observed to be strained in accordance with the structural mismatch to the underlying substrate, and this leads, in the thinnest films on LaAlO3, to an apparent monoclinic distortion to the unit cell.

Topics in Catalysis, 2007

Photochemical reactions that deposit insoluble products on catalytic surfaces have been used to p... more Photochemical reactions that deposit insoluble products on catalytic surfaces have been used to probe the anisotropy of the reactivity of SrTiO 3 microcrystals. Both reduced and oxidized products are formed preferentially on {100} surfaces. It is proposed that the anisotropic photochemical reactivity can be explained by the electronic band structure. Because direct optical transitions for charge carriers having momentum vectors in the <100> direction overlap well with the spectral distribution of the absorbed photons, more photogenerated carriers are moving toward {100} surfaces than other surfaces and, as a result, {100} surfaces are more active. Knowledge of the electronic band structure and the spectral distribution of the light allows predictions to be made about the anisotropic reactivity of photocatalysts with other crystal structures.

Thin Solid Films, 2013

The grain-by-grain orientation relationships between an Fe 2 O 3 film, grown using pulsed laser d... more The grain-by-grain orientation relationships between an Fe 2 O 3 film, grown using pulsed laser deposition, and a polycrystalline SrTiO 3 substrate were determined using electron backscatter diffraction. This high-throughput investigation, we call combinatorial substrate epitaxy, enables the characterization of film growth on all grain orientations in a single experiment, allowing the determination of the preferred epitaxial orientation (PEO) of this non-isostructural film/substrate pair. Heavily-twinned rhombohedral α-Fe 2 O 3 (hematite) grew epitaxially over the entire orientation space of the cubic perovskite substrate. Over 500 local orientation relationships (ORs) were investigated and more than 90% of these ORs, regardless of the interface plane normal, could be described using a single epitaxial OR: 0001 ð Þ 1010 h i Fe2O3 111 ð Þ 110 h i SrTiO3. This OR aligns the eutactic (nearly close-packed) planes and directions between these dissimilar crystal structures. Importantly, the growth of Fe 2 O 3 on a single crystalline (100)-SrTiO 3 results in several different orientation relationships. These results suggest that growth on high Miller-index (low-symmetry) surfaces provides more general information about the PEO than growth on low Miller-index (high-symmetry) surfaces. The epitaxial film growth on high Miller-index surfaces and the overwhelming observation of the eutaxial OR support the hypothesis that a very small number of simple crystallographic descriptors guide epitaxial film growth over all of orientation space, even for non-isostructural film/substrate pairs.

Thin Solid Films, 2006

Epitaxial hexagonal YMnO3 (h-YMnO3) films having sharp (00l) X-ray diffraction peaks were grown a... more Epitaxial hexagonal YMnO3 (h-YMnO3) films having sharp (00l) X-ray diffraction peaks were grown above 700° C in 5 mTorr O2 via pulsed laser deposition both on as-received wurtzite GaN/AlN/6H-SiC (001)(w-GaN) substrates as well as on w-GaN surfaces that were etched ...

Surface Science, 2003

We have examined the growth of thin Pt films on the polar SrTiO 3 (111) surface using both experi... more We have examined the growth of thin Pt films on the polar SrTiO 3 (111) surface using both experimental and theoretical methods. Pt films were grown on both as-received and treated substrates in a UHV chamber using electron beam evaporation, and they were characterized with x-ray and low-energy electron diffraction and atomic force microscopy. The nature of the substrate surface strongly influences the crystallinity and microstructure of the Pt films. We find that for well-treated, crystalline SrTiO 3 (111) substrates we obtain crystalline Pt(111) films. In addition, the first few layers strongly replicate the surface structure of the substrate. XRD results demonstrate that even the best films exhibit two distinct in-plane domains separated by 60 o , equivalent to the existence of inversion domains. DFT calculations were performed for 1 to 4 monolayers of Pt on both terminations of SrTiO 3 (111). These calculations find that two energetically equivalent stacking sequences are possible for the second monolayer of Pt, corresponding to the observed 60° domains. We propose that these stacking faults are the source of the observed domains in crystalline Pt(111) films grown on polar SrTiO 3 (111) surfaces. Finally, the DFT calculations corroborate the observation that the structures and growth modes of the first few monolayers are coupled strongly to the substrate, yet the films quickly relax to bulk-like Pt arrangements and homoepitaxial growth modes.

Surface Science, 2007

Low-energy electron diffraction (LEED), atomic force microscopy (AFM), and X-ray diffraction (XRD... more Low-energy electron diffraction (LEED), atomic force microscopy (AFM), and X-ray diffraction (XRD) have been used to investigate the structural and morphological character of a naturally chiral ceramic SrTiO 3 (621) substrate and of Pt and Cu thin films deposited on its surface. AFM experiments showed that as-received chirally-oriented SrTiO 3 (621) substrates display atomically smooth surface morphologies, while LEED patterns revealed that the surface structure has a net chirality. Pt(621) and Cu(621) thin films were grown heteroepitaxially on SrTiO 3 (621) substrates, as confirmed by XRD. AFM showed that the film surfaces were atomically smooth and LEED illustrated that the Pt films exhibit surface chirality, and by implication that the atomically-flat chirally-oriented Cu films also have chiral surfaces. The characteristics of the observed LEED patterns, where splitting of diffraction spots is considered to arise from the kinked step features of naturally chiral fcc metal surfaces, are discussed with respect to existing models. These results indicate that the chiral SrTiO 3 (621) ceramic surface drives the growth of single-enantiomer, chiral, metal (621) thin films.

Physical Review Letters, 2011

To study equilibrium changes in composition, valence, and electronic structure near the surface a... more To study equilibrium changes in composition, valence, and electronic structure near the surface and into the bulk, we demonstrate the use of a new approach, total reflection inelastic x-ray scattering, as a sub-keV spectroscopy capable of depth profiling chemical changes in thin films with nanometer resolution. By comparing data acquired under total x-ray reflection and penetrating conditions, we are able to separate the O K-edge spectra from a 10 nm La0.6Sr0.4CoO3 thin film from that of the underlying SrTiO3 substrate. With a smaller wavelength probe than comparable soft x-ray absorption measurements, we also describe the ability to easily access dipole-forbidden final states, using the dramatic evolution of the La N4,5-edge with momentum transfer as an example.

Nanoscale, 2014

The photocatalytic activity of materials for water splitting is limited by the recombination of p... more The photocatalytic activity of materials for water splitting is limited by the recombination of photogenerated electron-hole pairs as well as the back-reaction of intermediate species. This review concentrates on the use of electric fields within catalyst particles to mitigate the effects of recombination and back-reaction and to increase photochemical reactivity. Internal electric fields in photocatalysts can arise from ferroelectric phenomena, p-n junctions, polar surface terminations, and polymorph junctions. The manipulation of internal fields through the creation of charged interfaces in hierarchically structured materials is a promising strategy for the design of improved photocatalysts.

Journal of the American Ceramic Society, 2006

The photochemical properties of TiO 2 films supported on Ba-TiO 3 were investigated to test the h... more The photochemical properties of TiO 2 films supported on Ba-TiO 3 were investigated to test the hypothesis that dipolar fields from a ferroelectric substrate would affect the reactivity of the supported film. Photochemical reaction products were formed on the TiO 2 surface in patterns that correspond to the underlying domain structure of BaTiO 3. As the film thickness increases from 10 to 100 nm, the titania more effectively screens the ferroelectric field, and the pattern of reaction products is obscured. It is concluded that dipolar fields from the ferroelectric substrate influence charge carrier transport in the film and spatially localize the reaction products.

$Research paper thumbnail of Orientation and Phase Relationships between Titania Films and Polycrystalline BaTiO3 Substrates as Determined by Electron Backscatter Diffraction Mapping$

Journal of the American Ceramic Society, 2010

Titania films have been grown on polycrystalline BaTiO 3 (BTO) substrates at 7001C by pulsed lase... more Titania films have been grown on polycrystalline BaTiO 3 (BTO) substrates at 7001C by pulsed laser deposition. Electron backscatter diffraction (EBSD) was used to determine grain orientations in the substrate before growth, and the phase and orientation of the supported films after growth. All BaTiO 3 grains within 261 of (001) were covered by anatase films with an orientation relationship of (001) Anatase ||(001) BTO and [100] Anatase ||[100] BTO. Rutile with a variety of orientations grew on BaTiO 3 grains with orientations closer to (110) and (111). EBSD mapping provides an efficient means for determining phase and orientation relationships of films over all orientation parameters.

Journal of Materials Chemistry, 2011

Heterostructures of thin titania films on BiFeO 3 substrates were grown by pulsed laser depositio... more Heterostructures of thin titania films on BiFeO 3 substrates were grown by pulsed laser deposition. The heterostructures, when excited by visible light with energies between 2.53 and 2.70 eV, photochemically reduce aqueous silver cations from solution in patterns that mimic the structure of the ferroelectric domains in the substrate. Under the same conditions, titania by itself reduces insignificant amounts of silver. The observations indicate that electrons generated in the substrate are influenced by dipolar fields in the ferroelectric domains and transported through the titania film to reduce silver on the surface.

Journal of Crystal Growth, 2008

The influence of substrate temperature, oxygen activity of the ambient gas, and substrate type on... more The influence of substrate temperature, oxygen activity of the ambient gas, and substrate type on the growth morphology, phase selection, and epitaxy of thin films in the LaTiO x (xE3.0 or 3.5) family were investigated. The films were deposited using pulsed laser deposition (PLD) from a La 2 Ti 2 O 7 target and were characterized using X-ray diffraction. In oxygen atmospheres, coupled with high deposition temperatures and use of (1 1 0)-oriented SrTiO 3 substrates, the growth of epitaxial films of (1 1 0)-layered perovskite La 2 Ti 2 O 7 is observed. However, under similar deposition conditions, on SrTiO 3 (1 0 0) substrates, no crystalline peaks were observed even at the higher temperatures. The reduction of Ti 4+ to Ti 3+ was achieved by the use of nitrogen atmospheres. This resulted in the formation of the cubic perovskite LaTiO 3 , on SrTiO 3 (1 1 0), SrTiO 3 (1 0 0), and LaAlO 3 (1 0 0) substrates.

Journal of Crystal Growth, 2001

We have investigated the effects of wet etching with a 3 : 1 mixture of HCl : HNO 3 and of anneal... more We have investigated the effects of wet etching with a 3 : 1 mixture of HCl : HNO 3 and of annealing at 8508C on the surface morphology of [1 1 1]-oriented SrTiO 3 single crystals. Atomic force microscopy is used to demonstrate that the surface morphology is a strong function of both etching and annealing time. All surfaces have step heights equal to integral or half-integral multiples of the (1 1 1) interplanar spacing. However, step bunching, non-regular step heights, granularity, inhomogeneous surface morphology, and etch pits are observed on many surfaces. A combination of etching and annealing leads to surfaces that are free of these irregularities and are characterized by well-developed stepterrace structures: the terraces are % 600 Å wide, the step heights are either 1.2 Å or 2.3 Å , which are a half or a whole (1 1 1) interplanar distance, respectively, and the step edges are parallel to a single h1 1 0i-type direction. The existence of these two step heights implies that the surfaces are terminated by both SrO 3 and Ti planes.

International Journal of Hydrogen Energy, 2013

Heterostructured powders composed of microcrystalline (mc-) BaTiO 3 and SrTiO 3 cores coated with... more Heterostructured powders composed of microcrystalline (mc-) BaTiO 3 and SrTiO 3 cores coated with nanostructured (ns-) TiO 2 shells were prepared using a solegel method. The influences of annealing temperature, coating thickness, cocatalyst loading, and core size on photocatalytic hydrogen production were experimentally determined. The amount of hydrogen produced depends on the annealing temperature, which influences the interface, phase composition, light absorption, crystallinity, mesoporosity, and surface area. The heterostructured powders produced more hydrogen than ns-TiO 2 alone when annealed between 500 C and 800 C. The amount of hydrogen produced by heterostructures with 100 e150 nm thick nanostructured titania coatings was greater than for thicker or thinner coatings. The optimum Pt loading was determined to be 1% by weight. Heterostructured powders consisting of mc-BaTiO 3 /ns-TiO 2 produce more hydrogen than those with nanosized BaTiO 3 cores, suggesting a size effect that is counter to the conventional relationship between catalytic activity and particle size.

CrystEngComm, 2013

A high-throughput processing-characterization method, called combinatorial substrate epitaxy (CSE... more A high-throughput processing-characterization method, called combinatorial substrate epitaxy (CSE), was developed that enables the investigation of epitaxial stabilization of metastable compositions in complex structures. To demonstrate the approach, we fabricated RE 2 Ti 2 O 7 (RE = Dy, Gd, Sm, La) in a polymorphic structure for which RE = Dy, Gd, and Sm are metastable and Dy 2 Ti 2 O 7 has not been previously observed. Dense sintered pellets of Sr 2 Nb 2 O 7 , which adopts the 110-layered perovskite (LP) structure, were prepared as substrates, polished flat, and characterized locally using electron backscatter diffraction (EBSD). Thin films of RE 2 Ti 2 O 7 were deposited using pulsed laser deposition and were then characterized with EBSD. The EBSD patterns from all film-substrate pairs matched in a grain-by-grain fashion, which demonstrates that the films are in local epitaxial registry with the Sr 2 Nb 2 O 7 grains over a wide spread of crystallographic orientations for the substrate surface. Furthermore, the EBSD patterns demonstrate that all RE 2 Ti 2 O 7 films, whether stable or metastable in the bulk, adopt the 110-LP structure. Transmission electron microscopy was used to investigate more closely the metastable Sm 2 Ti 2 O 7 films. The film-substrate interfaces are atomically smooth with relaxed epitaxial registry, indicating that the microcrystalline substrates can be treated as local single-crystal substrates and the metastable films are stable against back-transformation on strain relaxation. Electron diffraction patterns for Sm 2 Ti 2 O 7 films are consistent with the monoclinic 110-LP unit cells. This work demonstrates that CSE allows for the growth of new materials that are thermodynamically and kinetically difficult to realize otherwise.